1. Field of the Invention
The present invention relates to an AC powder type EL panel to be used as, e.g., a back light of a liquid crystal display device, an illumination light source, and a display element and a method of manufacturing the same.
2. Description of the Related Art
FIGS. 1A and 2 show the structure of a conventional AC powder type EL panel. FIG. 1A is a sectional views perpendicular to a light-emitting surface of the AC powder type EL panel, and FIG. 2 is a plan view showing the AC powder type EL panel viewed from the above the light-emitting surface. Referring to FIG. 1A, a reflective insulating layer 2 is formed on a backplate 1 consisting of an aluminum foil or the like, a light-emitting layer 3 is formed on the reflective insulating layer 2, and a transparent conductive film 4 is bonded by thermocompression on the light-emitting layer 3. The transparent conductive film 4 is constituted by a resin film 4b as a substrate consisting of, e.g., polyester and a transparent electrode 4a formed on the resin film 4b. The transparent film 4 is bonded by thermocompression on the light-emitting layer 3 so that the transparent electrode 4a faces down, thereby constituting the AC powder type EL element. The AC powder type EL panel is constituted by the AC powder type EL element as described above, a pair of moisture-trapping films 5 formed on the upper and lower surfaces of the AC powder type EL element and consisting of, e.g., nylon, thermoplastic adhesive layers 6b formed on the upper and lower surfaces of the moisture-trapping films 5, and a pair of protective films 6a having good moisture barrier properties and bonded by thermocompression from the above and below the pair of moisture-trapping films 5 via the thermoplastic adhesive layers 6b to seal the AC powder type EL panel.
As shown in FIG. 2, as the transparent electrode, a transparent conductive film 4 obtained by forming a thin film of a transparent electrode layer 4a on a resin film substrate 4b, and coating a silver paste of a bar-shape on the resulting thin film and baking it to form an auxiliary electrode 4c, can be used. Leads 7 consisting of phosphor bronze or aluminum a normally, externally led from the backplate 1 and the auxiliary electrode 4a formed on the conductive film 4.
With the above arrangement, light emission can be obtained from the EL light-emitting element by applying an AC electric field having about 100 V and 100 to 1,000 Hz across the leads 7. In this state, however, the light-emitting layer 3 absorbs moisture to deteriorate the phosphor. Therefore, a method of manufacturing this AC powder type EL element additionally requires a step of forming the protective films 6a as polymer films having good moisture barrier properties to seal the element and a step of forming the moisture-trapping layers 5 for trapping moisture permeating through the protective films 6a.
As the moisture-trapping layers 5, a pair of moisture-trapping films 5 having good moisture absorption characteristics such as nylon resin films are formed outside the AC powder type EL element. An adhesive is coated on one surface of each nylon resin film 5, and the films 5 are bonded to the AC powder type EL element by thermocompression by a laminator with the AC powder type E element being sandwiched between the films 5 such that the adhesive faces inside.
As the protective films 6a, films having good moisture barrier properties and small moisture permeability such as fluoroplastic films are used. The protective film 6a has a size larger than that of the AC powder type EL element. The thermoplastic adhesive layer 6b is coated on one surface of each protective film 6a. The protective films 6a are bonded by thermocompression to sandwich the AC powder type EL element such that the adhesive faces inside. The AC powder type EL panel has a structure in which portions of the protective films 6a extending from the AC powder type EL elements are bonded by thermocompression to each other by a laminator, thereby sealing the elements.
A laminator used in thermocompression bonding of the protective films 6a and the thermoplastic adhesive layers 6b is constituted by at least a pair of heat rollers having an internal heater. Sealing of the AC powder type EL elements are performed as follows. That is, a plurality of AC powder type EL elements are aligned between two opposing elongated protective films such that distal end portions of their lead extend from the protective films, and the two protective films are bonded by thermocompression to each other. The upper and lower protective films and the thermoplastic adhesive layers integrated by sealing are cut into a predetermined size by a press cut method, thereby manufacturing an AC powder type EL panel.
The A powder type EL panel obtained by the above manufacturing method, however, has a problem of uneven deterioration of a light-emitting layer caused by penetration of moisture from a peripheral portion of the laminated protective film. When this uneven deterioration occurs, a distribution of brightness of the AC powder type EL panel is significantly deteriorated within a short time period. Therefore, when the AC powder type EL panel having the uneven deterioration is used as a back light of a liquid crystal display, it is difficult to read displayed characters.
The uneven deterioration of the light-emitting layer is mainly caused by penetration of moisture from the thermoplastic adhesive layers formed on the protective films. As described above, the protective films are bonded by thermocompression from the above and below the AC powder type EL elements via the thermoplastic adhesive layers to seal the elements and cut into a predetermined shape by a press cut method or the like. This cut surface is shown in an enlarged scale in FIG. 1B. As shown in FIG. 1B, the thermoplastic adhesive layers are exposed to the cut surface between the upper and lower protective films. External moisture permeates the exposed thermoplastic adhesive layers and penetrates into the panel. The light-emitting layer at the peripheral portion of the light-emitting surface is rapidly deteriorated by the penetrating moisture to cause uneven deterioration of the light-emitting surface. Therefore, a strong demand has arisen for development of an AC powder type EL panel which improves moisture barrier properties of the protective films and the thermoplastic adhesive layers to prevent uneven deterioration of the light-emitting layers.
As described above, according to a conventional AC powder type EL panel obtained by vertically sandwiching AC powder type EL elements by protective films having a larger size than that of the elements via thermoplastic adhesive layers, performing thermocompression bonding to seal the AC powder type EL elements by a laminator, and cutting the protective films and the thermoplastic adhesive layers into a predetermined size, if cutting of the protective films and the thermoplastic adhesive layers is performed by a press cut method, the thermoplastic adhesive layers between the thermocompression-bonded protective films are exposed to the cut surface. Therefore, moisture outside the panel penetrates into the panel through the thermoplastic adhesive layers to cause uneven deterioration in the light-emitting layer from the peripheral portion of the light-emitting surface.